There is known a magnetic random access memory (referred to as MRAM herein after) which stores data by controlling magnetization directions of storage elements. There are several types of MRAMs corresponding to recording methods of magnetization directions.
U.S. Pat. No. 6,545,906 discloses a toggle-type magnetic random access memory (referred to as a toggle MRAM herein after). In this toggle MRAM, a storage element is a tunnel magnetoresistive element with a synthetic ferrimagnet structure used as a free layer. This MRAM shows excellent selectivity of a memory cell at a time of a write operation. The details of the toggle MRAM will be explained below.
FIG. 1 is a cross-sectional view showing a structure of a conventional toggle MRAM. A magnetoresistive element 105 of a memory cell 110 in this MRAM includes an antiferromagnetic layer 104, a synthetic ferrimagnet pinned layer 103, a nonmagnetic layer (or tunnel insulation layer) 102 and a synthetic ferrimagnet free layer 101. Each of the layers is laminated in this order. The synthetic ferrimagnet pinned layer 103 has a synthetic ferrimagnet structure, including a ferromagnetic layer 116, a nonmagnetic layer 115, and a ferromagnetic layer 114. The synthetic ferrimagnet free layer 101 has a synthetic ferrimagnet structure, including a ferromagnetic layer 113, a nonmagnetic layer 112 and a ferromagnetic layer 111. The memory cell 110 stores data based on a magnetization direction 122 of the ferromagnetic layer 113 and a magnetization direction 121 of the ferromagnetic layer 111. The magnetoresistive element 105 is held between a write word line 126 and a bit line 127 which are crossed from one another in a substantially perpendicular state.
FIG. 2 is a top surface view showing a structure of the conventional toggle MRAM. In this MRAM, the plurality of write word lines 126 and the plurality of bit lines 127 are arranged perpendicularly to each other (though only one of each is shown in FIG. 2), and the magnetoresistive elements 105 are arranged at cross points made by these two lines. The magnetoresistive element 105 is arranged so that an easy magnetization direction (i.e. easy magnetization axis) is pointed at about a direction of 45 degrees (θ) with respect to the word line 126 and the bit line 127. It is in order to realize an easy toggle operation.
Next, a principle of a write operation will be explained in the conventional toggle MRAM. The toggle MRAM is allowed to write only from “1” to “0” and “0” to “1”. That is, it is impossible to overwrite “1” to “1” and “0” to “0”. Therefore, in a write operation of the conventional toggle MRAM, data is read from a selected memory cell 110 (referred to as a selected cell herein after) in advance. Then, the write operation is not carried out if the read data (“0” or “1”) is equal to data to be written (“0” or “1”), or the write operation is carried out by the toggle operation (i.e. toggle writing) if the read data is different from the data to be written.
FIGS. 3A to 3H show a principle of a toggle operation in a conventional toggle MRAM. FIG. 3A is a timing chart of a write current IBL which is made to flow in the bit line 127. FIG. 3B is a timing chart of a write current IWL which is made to flow in the word line 126. FIG. 3C shows a time variation in a magnetization direction 122s of the ferromagnetic layer 113 (shown by thick arrows) and a magnetization direction 121s of the ferromagnetic layer 111 (shown by thin arrows) in a selected cell. FIG. 3D shows a time variation in directions of magnetic fields generated by the write current IBL and the write current IWL. FIG. 3E shows time variations in a magnetization direction 122a of the ferromagnetic layer 113 (shown by thick arrows) and a magnetization direction 121a of the ferromagnetic layer 111 (shown by thin arrows) in a non-selected cell which is disposed on the same bit line 127 as the selected cell. FIG. 3F shows a direction of a magnetic field 123 generated by the write current IBL. FIG. 3G shows time variations in a magnetization direction 122b of the ferromagnetic layer 113 (shown by thick arrows) and a magnetization direction 121b of the ferromagnetic layer 111 in a non-selected cell which is disposed on the same word line 126 as the selected cell. FIG. 3H shows a direction of a magnetic field 125 generated by the write current IWL.
Referring to FIG. 3A, in the toggle operation, the write current IBL is supplied to the bit line 127 at time t2. The write current IWL is supplied to the word line 126 at time t3. The write current IBL is discontinued at time t4. The write current IWL is discontinued at time t5. Owing to the series of the above current controls, magnetic fields such as the magnetic field 123 to a magnetic field 124 to a magnetic field 125 as shown in FIG. 3D are added to a selected cell disposed in a cross point between the selected word line 126, to which the write current IWL is supplied, and the selected bit line 127, to which the write current IBL is supplied. Therefore, the magnetization direction 122s of the ferromagnetic layer 113 and the magnetization direction 121s of the ferromagnetic layer 111 are rotated in the selected cell as shown in FIG. 3C, whereby data can be written. That is, an initial state of “0” is rewritten (or toggled) to a state of “1” or an initial state of “1” is rewritten (or toggled) to a state of “0”.
At this time, only a unidirectional magnetic field such as the magnetic field 123 as shown in FIG. 3F is added to a non-selected cell which is disposed on the same bit line 127 as the selected cell. Therefore, as shown in FIG. 3E, the magnetization direction 122a of the ferromagnetic layer 113 and the magnetization direction 121a of the ferromagnetic layer 111 in the non-selected cell exhibit some variations but return to an original state without writing data in the non-selected cell. Similarly, only a unidirectional magnetic field such as the magnetic field 125 as shown in FIG. 3H is added to a non-selected cell which is disposed on the same word line 126 as the selected cell. Therefore, as shown in FIG. 3G, the magnetization direction 122b of the ferromagnetic layer 113 and the magnetization direction 121b of the ferromagnetic layer 111 in the non-selected cell exhibit some variations but return to an original state without writing data in the non-selected cell. Accordingly, toggle writing is capable of preventing multiple writing in which data to be written in the selected cell is also written in the non-selected cell.
An other advantage of toggle writing is that only a unipolar voltage needs to be generated because a direction of a write current is unidirectionally fixed. As a result, smaller transistors can be used as transistors for writing.
However, it is impossible to sort and write “1” and “0” in the conventional toggle MRAM. Therefore, even in a writing cycle, the data needs to be read once to determine whether or not to write data, followed by writing the data. As a result, there is a disadvantage that the writing circle is prolonged due to a period of data reading time and a writing speed is consequently delayed. Since the writing speed is improved if the data reading is omitted in the write circle, there is a demand for a technique to sort and write “1” and “0”.
U.S. Pat. No. 6,545,906 discloses a direct writing method as a technique to sort and write “1” and “0”. The direct writing method is realized such that a total magnetic moment of two ferromagnetic layers composing a synthetic ferrimagnet free layer is switched in response to an external magnetic field which is equal to or larger than a critical magnetic field. In this method, magnetization is arranged so as to direct the total magnetic moment to the direction to which the magnetic field is applied, whereby “1” and “0” can be written without reading data in advance. This method has a disadvantage in that the multiple writing tends to occur. An other disadvantage of this method is that a bipolar voltage is required to cause write currents to flow to dual directions, which results in requiring a large transistor as a transistor for writing.
Related techniques include Japanese Laid-Open Patent Application JP-A-Heisei 11-273336 (corresponding to U.S. Pat. No. 5,946,228) which discloses an elongated MRAM cell provided with a central nucleus formation for switching. This magnetic device includes a first magnetic region and a magnetic application structure. The first magnetic region can be changed into two magnetic states in response to magnetic stimuli applied thereto. The magnetic application structure is arranged with respect to the first magnetic region in order to apply the magnetic stimuli only to a preferred portion of the first magnetic region. The elongated first magnetic region is formed so as to maintain a common magnetization direction at the respective end portions thereof while having either of the two substantially opposing magnetization directions in the center portion thereof, or the first magnetic region may include a pinned magnetization source at the respective end portions thereof.
U.S. Pat. No. 6,798,690 discloses a magnetoresistive memory. The magnetoresistive memory includes a nonmagnetic layer sandwiched between two magnetic layers. The magnetic layer includes a first bit end having an expanded magnetic volume for supporting magnetization of the first end along a hard magnetization axis, a second bit end having an expanded magnetic volume for supporting magnetization of the second end along the hard magnetization axis, and a body interconnecting the first and second bit ends for supporting body magnetization along an easy magnetization axis. The first and second bit ends may be arranged in an I-shape configuration.
Japanese Translation of PCT No. 2003-518699 (corresponding to WO00/007191) discloses a method of reading/writing an MRAM array. This method of writing and/or reading a magnetic memory array includes the steps of: providing an array of magnetic cells; applying an initialization hard axis magnetic field to the array in a first direction; and selecting a cell in the array for writing. Here, each magnetic cell has an easy magnetization axis, a hard magnetization axis and end domain magnetization, and the array has a first current line that generates an easy axis magnetization field and a second orthogonal current line (12) that generates a hard axis magnetization field when currents are applied thereto. The initialization hard axis magnetization field switches or maintains end domain magnetization field in all cells in a pinned direction. Selecting a cell in the array includes supplying a current to the first current line associated with the selected cell to generate an easy axis magnetization field, simultaneously supplying a current to the second current line associated with the selected cell to generate a hard axis magnetization field, and supplying a current to the second current line which generates a hard axis magnetization field in the first direction.
Japanese Laid-Open Patent Application JP-P 2003-163330 A discloses a magnetic memory. This magnetic memory includes: a first magnetoresistive effect element; a first wiring extended above a first magnetoresistive effect element; a second magnetoresistive effect element arranged above the first wiring; and a second wiring extended in a direction to cross the first wiring above a second magnetoresistive effect element. Each of the first and second magnetoresistive effect elements includes magnetic recording layers with substantially unidirectional magnetization anisotropy. Magnetization of the magnetic recording layer in the second magnetoresistive effect element can be switched owing to a magnetic field created by currents which are made to flow in the first and second wirings, respectively. At least one part of magnetization directions of the magnetic recording layers in the first and second magnetoresistive effect elements are partially inclined to at least any of the first and second wirings.
Japanese Laid-Open Patent Application JP-P 2003-332537 A discloses a magnetic memory element and a magnetic random access memory using the same. This magnetic memory element includes at least two layers of ferromagnetic layers laminated via a nonmagnetic intermediate layer, where information is recorded by changing a magnetization direction of at least one of the ferromagnetic layers between which the nonmagnetic intermediate layer is held, and the state is read by magnetoresistive effects. Then, at least a part of the plane shape of the magnetic memory element includes a cycloid or a cycloidal curve.